1. Field of the Invention
This invention relates to apparatus for controlling and adjusting sail shape, and particularly to a structure including a curved rigid spar which is arranged to reverse its curvature when tacking and which may be set at a controlled rotation as a draft control device.
2. Description of Prior Art
A problem fundamental to sailing and sailmaking is the control of sail shape. Basically, this amounts to designing a sail and rig (spars and control devices) with sufficient adjustability so that a sail can have a full, powerful, (large camber ratio) shape in light winds, or when reaching and running, and yet, be able to be progressively flattened, or "de-powered", as wind increases, or for sailing to weather (upwind). This is accomplished in mainsails by mechanically bending, or straightening, the mast, by adjusting the clew outhaul in or out, and by adjusting the sail in or out from the centerline of the boat. This generally works well, but does not do an entirely satisfactory job of controlling fully battened mains with large roach (excess cloth at leach). Here, increasing wind strength causes the battens to bend more, and the roach tends to move forward, both of which increase the draft of the sail when draft reduction is desired. With headsails in increasing wind, draft can be reduced to a point by increasing the luff tension in the sail, by sheeting the sail harder and moving the sheet lead outboard, and by increasing headstay tension to reduce luff sag. Ultimately though, these methods fail as wind strength increases to the point where headstay sag cannot be kept within the limits the sail was designed for. Then, a flatter (cut for more sag) and usually smaller headsail must be substituted. The converse is true with decreasing winds. Also, as wind strength increases, woven sailcloth tends to stretch in ways that increase sail draft. It should also be noted that present rig control systems require sailcloth stretch characteristics that are difficult to produce in new cloth, and are seldom present in sails that have been used a season or two.
A further sail shape control problem is the control of sail twist (angle of sail cord to centerline of boat at different elevations above the deck). While mainsail twist is controlled fairly well by travelers and vangs, little has been done to control twist in headsails when reaching or running.
The conventional method used to spread the foot of a mainsail, and some non-overlaping headsail is by attaching the foot of the sail to a boom, which is a straight spar pivoted at or near the forward lower corner (tack) of the sail. Another method, once used on small craft, was to run a straight spar from the clew of the sail to the mast, or forestay a few feet above the tack of the sail. This had an advantage in that it formed a structural triangle (spar and mast in compression and foot of sail in tension for mainsails) which prevented the clew of the sail from rising, and hence the sail from twisting excessively, when the sheet was eased. This then kept the sail in shape better than a boom attached to the foot of the sail. The disadvantage was that the airfoil shape of the sail was disrupted by this spar on one tack. To solve this shortcoming, Nathaniel Herreshoff and others devised the wishbone boom early in this century. This consisted of a pair of curved spars that surround the mast and sail. With this arrangement, good sail control was achieved on both tacks, but there was a great deal of clutter and windage (parasitic drag) associated with this rig. This rig has been revived in recent years for some production boats such as the Freedom Ketches, the Stone Horse Sloop, and the Nonsuch Catboats. There is a simplification of the wishbone rig (by Phil Bolger and others) whereby only half of the wishbone is used. This half wishbone remains on one side of the sail and is supported by multiple topping lifts. It is always curved in the same direction, so that on one tack it lies close to the sail, and on the other tack it is bowed away from the sail. Even with this rig there is still a lot of windage.
Also, a conventional method of stiffening sails (usually for high speed sails for iceboats, land yachts, and multihulls) is to insert battens into batten pockets that run from the luff to the leech of the sail. These so called-full length battens are straight pieces of wood or fiberglass, or some similar material. Wind pressure is required to make the battens bend and allow the sail to assume a good shape. This generally works very well, but occasionally in light air the battens will not bend enough, and the heavy air shortcomings were noted above. Also, because the battens must be flexible, they are subject to breakage.